_scan_placeholders shouldn't do any magic to get an ``include`` template.

Hard-coded ``.strip('"')`` meant server exceptions when using single quotes for the include template name e.g. ``{% include '_messages.html' %}``. Should use the already parsed ``node.template.var`` directly.
Merge pull request #3441 from jsma/template-issue

_scan_placeholders shouldn't do any magic to get an ``include`` template...